The present invention relates to the field of treating aqueous salt solutions, and more particularly, the invention relates to controlling the salt stream to be treated in an electrodialytic water splitter.
Electrodialytic processes are used to recover acids and bases from materials comprising soluble salt. The process for recovering acid uses an electrodialytic water splitter to regenerate acid and a dilute salt solution. This process is useful to regenerate spent process materials for reuse rather than disposing of them.
Pickling baths, for example, are employed to remove the scale, oxides and other impurities from metal surfaces such as stainless steel. These baths comprise inorganic acids such as hydrochloric acid, sulfuric acid, phosphoric acid, nitric acid, and hydrofluoric acid, and commonly are mixtures thereof. In use, the bath gradually becomes exhausted due to the reaction of the acids with the oxides, scale, etc. Eventually, the bath is converted to a spent solution comprising salts and some free (unreacted) acid. This spent solution must .then be disposed and the acid must be replaced at a cost which can be very substantial.
The economic impact of disposing of the spent process materials can be as expensive as the cost of replacing the acid bath. Moreover, the environmental impact of disposing of these materials is of significant concern. One method of disposing of spent pickling liquor requires neutralizing the liquor with lime to precipitate the metal values and fluorides, and then landfilling the solid waste, with the remaining solubles being discharged after further treatment. However, this disposal process is very expensive and, because of the toxicity of the residue, especially if it contains fluoride ions, significant environmental damage can occur if it is improperly disposed.
Processes are known for the recovery of free acid from materials containing the same. They include (a) diffusion dialysis, (b) acid retardation using ion exchange resins, and (c) "the Ruthner process". Diffusion dialysis is the process of removing free acid from a liquid by transporting acid from a compartment containing the liquid across a membrane to adjacent compartments containing water, where the transport driving force is the concentration gradient of acid across the membrane. Acid retardation is the process of absorbing free acid from the material by use of an ion exchange resin, removing the ion exchange resin containing the acid from the material, and then washing the ion exchange resin with water to desorb the acid. "The Ruthner process" is the process of isolating free acid from a liquid via an evaporative crystalization process.
Each of the above mentioned processes has major shortcomings. With diffusion dialysis, acid recovery diminishes as the concentration gradient is reduced. With acid retardation using ion exchange resin, acid recovery is limited by the absorption and desorption capacity of the resin. Moreover, both processes have the added problem of disposing of the remaining acid-depleted salt stream. With "the Ruthner process" the regenerated acid normally contains five to ten times the metal content as the acid produced by the above mentioned processes. Moreover, "the Ruthner process" is energy intensive and highly corrosive to the process equipment.
Electrodialytic water splitting processes for regenerating acids and base from salts are known. For example, in U.S. Pat. Nos. 4,082,835 and 4,107,015, processes are disclosed for regenerating scrubbing solutions used in stripping SO.sub.x from flue gases by feeding salt-containing product solutions from the stripping step through an electrodialytic water splitter. Also, in U.S. Pat. No. 4,504,373, a process is disclosed for regenerating a dilute sulfuric acid solution for use in the processing of rayon from a spent rayon spin bath containing a sulfate salt by subjecting the salt to electrodialytic water splitting.
In U.S. Ser. No. 196,829, filed May 16, 1988 to Chlanda et al., commonly assigned, a process for recovering mixed acids from mixed salts at high current efficiency is disclosed. The process comprises the steps of providing an electrodialytic water splitter comprising at least one unit cell, each unit cell defining at least two compartments, feeding an aqueous solution comprising at least two salts formed from at least two different anions to one compartment, one of said anions being fluoride; feeding a liquid comprising water to another compartment; passing current through said electrodialytic water splitter to produce an aqueous product comprising mixed acids formed from the different anions in one compartment.
U.S. Pat. No. 4,740,281 discloses a recovery process that combines electrodialysis with electrodialytic water splitting. The process solution comprising acid and salt is initially subjected to electrodialysis in an apparatus employing cation and anion permselective membranes to recover the free acid from the process solution. During the operation of the electrodialysis step, the hydrogen ions of the free acid are transported across cation permselective membranes from a compartment containing the process solution to a compartment in which the hydrogen ions combine with anions to form acid which can be recovered. The remaining acid depleted product, mainly comprising metal salt, is recovered and is then subjected to electrodialytic water splitting to convert the salt into acid and base. The acid from the electrodialytic water splitting step can be combined with the recovered acid from the electrodialysis step and reused, further concentrated, sold, etc. At least part of the depleted salt solution from the electrodialytic water splitter is supplied to a dilute salt solution holding tank. The weak salt solution is further depleted in an electrodialysis unit having a diluting compartment and a concentration compartment. The depleted salt is fed to the diluting compartment and is further diluted. The concentrated product from the concentration compartment is supplied to a filtrate tank where it is combined with the salt solution at least part of which is fed to the salt compartment of the electrodialytic water splitter.